Circuit arrangement to detect a voltage

ABSTRACT

In a circuit arrangement to detect a voltage (U), the voltage is applied to the input of a voltage detector (VD) through a voltage divider (R1, R4), the voltage detector (VD) comprising a Schmitt-Trigger with reference voltage and an output driver. The output signal of the voltage detector (VD) is applied to the base of a transistor (T1) whose collector-emitter circuit, together with a series-connected capacitor (Cl), is connected in parallel with the voltage (U) to be detected. The junction of the capacitor (C1) and the transistor (T1) is connected to the input of the voltage detector (VD) through a resistor (R5, R6). The output of the voltage detector (VD) is further applied to the input of a component (C) delivering different output signals in dependence upon whether its input receives a constant signal level or a varying signal level.

This invention relates to a circuit arrangement to detect a voltage,including a voltage divider having applied to its divider node the inputof a voltage detector which comprises a Schmitt-Trigger with referencevoltage and an output driver, with the output of the voltage detectorbeing connected to the positive potential of the voltage.

Voltage detectors comprising internally a Schmitt-Trigger with referencevoltage and an output driver are commercially available devices. FIG. 4illustrates the wiring of such a voltage detector VD when a specifiedvoltage, for example, the voltage of two serially connected batterycells or accumulator cells (B), is to be detected which is greater thanthe internal reference voltage of the voltage detector VD. In thisevent, a voltage divider comprised of resistors R1 and R4 is connectedin parallel with the battery B, and the junction of the two resistors R1and R4 is connected to the IN input of the voltage detector VD. The OUToutput of the voltage detector VD is applied to the positive potentialof the battery B through a resistor R2. The voltage divider R1/R4 is sodimensioned that the internal reference voltage (2.1 volts, for example)will be present at the IN input of the voltage detector VD when thevoltage to be detected (2.3 volts, for example) is present at thebattery B. When the battery voltage drops below-this value, the outputof the voltage comparator VD will go from "high" to "low".

However, such commercially available voltage detectors have thedisadvantage of having a hysteresis of, for example, 0.1 volts. Thismeans that with the battery voltage falling the output of the voltagedetector VD will go from "high" to "low" when the voltage at the inputreaches 2.1 volts (correspondingly 2.3 volts at the battery), however,with the battery voltage rising (for example, when the battery is beingrecharged), the output of the voltage detector VD will not reverse itsstate until the voltage at the input reaches 2.2 volts (2.1 volts+0.1volts). This is not satisfactory for some applications requiring anaccurate voltage detection as, for example, when it is desired to detecta specified voltage accurately on a battery during both discharging andcharging of the battery.

It is therefore an object of the present invention to configure acircuit arrangement of the type initially referred to in such a manneras to permit a hysteresis-free detection of a specified voltage.

According to the present invention, this object is accomplished in thatthe output signal of the voltage detector is applied to the base of afirst transistor, that the collector-emitter circuit, together with aseries-connected capacitor, is connected in parallel with the voltage tobe detected, that the junction of the capacitor and the first transistoris connected to the input of the voltage detector through a firstresistor, and that the output of the voltage detector is further appliedto the input of a component delivering different output signals independence upon whether its input receives a constant signal level or avarying signal level.

When it is desired to detect several specified voltages using the samevoltage detector or to be able to vary the value of the voltage to bedetected, the resistor of the first voltage divider connected toreference potential is comprised of a second voltage divider to which anintegrating capacitor is connected in parallel and whose divider nodereceives a square-wave voltage whose pulse duty factor is variable.

Further advantageous embodiments of the present invention will becomeapparent from the other subclaims and the subsequent description.

Embodiments of the present invention will now be described in moredetail in the following with reference to the accompanying drawings, inwhich:

FIG. 1 is a schematic diagram showing a circuit arrangement fordetecting a specified voltage value on a battery;

FIGS. 2 and 3 are schematic diagrams showing circuit arrangements fordetecting several voltage values using a single voltage detector; andFIG. 4 is a schematic diagram of a commercially available voltagedetector.

Referring now to FIG. 1 of the drawings, there is shown a circuitarrangement providing a suitable indication when the voltage U of thebattery B has reached a specified value. A load resistor RL may beconnected to this battery (accumulator) through a switch S. This loadresistor may be the motor of a small electrical appliance as, forexample, an electric shaver. The battery is rechargeable by means of acharging circuitry not shown.

Connected in parallel with the battery B is a voltage divider comprisedof resistors R1 and R4, by means of which the voltage U of the battery Bis divided down to the voltage Ue residing at the junction of theresistors R1 and R4 which is applied to the IN input of the voltagedetector VD. The OUT output of the voltage detector VD is connected tothe positive terminal of the battery B through a resistor R2, to thebase of a transistor T1 through a resistor R3, and to the microprocessorC driving a display device D through a resistor R10.

The emitter of transistor T1 is connected to the positive terminal ofthe battery B, its collector being connected to reference potentialthrough a capacitor C1. In addition, the collector is connected to thejunction of the resistors R1 and R4 of the voltage divider throughresistors R5 and R6. The resistor R6 is of the variable type foradjustment.

The voltage divider R1/R4 is dimensioned such as to ensure that, whenthe battery B reaches a specified voltage U, the divided-down voltage Ueat the IN input of the voltage detector VD lies below the detectionvoltage of the voltage detector VD. This specified voltage U may be, forexample, the "low charge" point at 2.3 volts (in which event the batteryis discharged to 10% to 20% of its capacity). When the detection voltageof the voltage detector is, for example, 2.1 volts and the tolerance is±0.1 volts (which has no relation to the hysteresis of the voltagedetector), these are 2.0 volts.

The mode of function of the circuit arrangement is as follows: When thebattery voltage U has dropped to the voltage to be detected which is,for example, 2.3 volts, the voltage Ue at the IN input of the voltagedetector VD will be 2.0 volts, and the OUT output of the voltagedetector VD will change from "high" to "low". "Low" is the active stateof the voltage detector VD. Transistor T1 will then conduct, causing theseries arrangement comprised of the resistors R5, R6 to be connected inparallel with resistor R1. The input voltage Ue (divider voltage) isthereby increased to a value greater than the detection voltage of thevoltage detector plus the hysteresis voltage, causing the voltagedetector to assume the reverse state again (release voltage) and the OUToutput to return to "high". The transistor T1 is again non-conducting,cancelling the parallel connection of resistors R5, R6 to resistor R1,whereby the divider voltage Ue is again below the detection voltage ofthe voltage detector VD, and the OUT output is again changed to "low".

The circuitry oscillates. The capacitor C1 provides time delays for thetransition operations, thus reducing the oscillation frequency to about1 kHz, for example. A square-wave voltage with an amplitude of the orderof the battery voltage U is present at the OUT output.

Oscillation will cease to be possible when the release voltage of thevoltage detector is no longer attained with the transistor T1conducting. At this instant, the state of the voltage detector VD willbe maintained, its output voltage being constantly at "low". Thisvoltage point is adjusted by means of resistor R6.

Thus, it is possible to detect a specified value of the supply voltagesolely by the presence of a constant signal level at the output of thevoltage detector VD with respect to a varying (oscillating) voltagelevel. The specified voltage value is detected without hysteresis, thatis, it is irrelevant whether the specified voltage value is reachedstarting from higher or from lower voltage values, since it is not theinternal reference voltage of the voltage detector (2.1 volts) that isused for adjustment, but rather its release voltage.

The circuit arrangements of FIGS. 2 and 3 present an extension of FIG. 1to cover the detection of several voltages using just a single voltagedetector VD. In these Figures, the resistor R4 of FIG. 1 is subdividedinto serially connected resistors R7, R8 and R9, whereof resistor R7 isconnected to the input of the voltage detector VD, while resistor R9 istied to reference potential. An integrating capacitor C2 is connected inparallel with the series arrangement comprised of resistors R8 and R9.

In FIG. 2, an output of the microprocessor C is connected to thejunction of resistors R8 and R9. The microprocessor supplies to thisjunction a square-wave control signal whose pulse duty factor isvariable.

Owing to this variable pulse duty factor, in combination with theresistors R7, R8, R9 and the integrating capacitor C2, the releasevoltage point of the voltage detector VD can be shifted over a widerange of the voltage U of the battery B. The greater the pulse dutyfactor of the square-wave voltage supplied, that is, the greater thepulse/no-pulse ratio, the higher the mean value of the voltage supplied.Correspondingly, the voltage at the junction of capacitor C2 andresistor R8, and consequently also the voltage Ue at the input of thevoltage detector VD, are raised, whereby the release voltage of thevoltage detector is reached at higher battery voltages U.

As described with reference to FIG. 1, it is thus possible to determineand detect any desired number of voltage points of the battery voltage Uby varying the pulse duty factor of the square-wave voltage supplied. Bysensing a voltage point and subsequently specifying a next voltage pointby correspondingly varying the pulse duty factor of the square-wavevoltage supplied, the voltage characteristic of a battery duringcharging and/or discharging can be retraced in dependence on time.

By allocating specified voltage points of a discharging or chargingcharacteristic of the battery (for example, between 2.5 volts and 2.2volts at 50 mV intervals) to a visual signal of the display device D,the current charging condition of the battery is displayed. Where adisplay device D comprising several segments is employed, the segmentsare controlled by the microprocessor C such that in the fully chargedcondition (highest detected voltage), all segments are driven, whilstwith the battery charge near depletion (lowest detected voltage), nosegment is driven.

Where the display device D provides a continuous indication of thecharging condition on a time basis, that is, in dependence on the periodof time during which the load resistor RL (load) was connected to thebattery or the battery was charged, the voltage points detected inoperation will be compared to the values of the battery charging anddischarging characteristic stored in the microprocessor, and if adeviation is established, the display will be corrected correspondingly.

FIG. 3 shows a circuit arrangement suitable for the event that thevoltage divider is not sufficiently high-resistance, that is, the outputof the microprocessor C supplying the square-wave voltage is not in aposition to provide the requisite power. In this case, the square-wavesignal of the microprocessor C will be delivered through a resistor R11to the base of a transistor T2 whose collector-emitter circuit is inparallel with the resistor R9. In accordance with the pulse duty factorof the square-wave signal supplied, the transistor T2 is rendered eitherconducting or non-conducting. In all other respects, the mode offunction of the circuit arrangement is the same as that of FIG. 2.

All circuit arrangements described in the foregoing areself-oscillating, requiring no control voltages from an external source.In consequence of the low current requirements, the circuit arrangementsmay remain connected to the supply voltage U also with the applianceturned off (load resistor RL disconnected from the battery), without thebattery discharging unacceptably.

I claim:
 1. A circuit arrangement to detect a voltage (U), comprising atransistor having a base and a collector-emitter circuit, a capacitorconnected in series with said collector-emitter circuit, a voltagedetector that has an input and an output, circuitry connecting saidvoltage detector output to the positive potential of said voltage (U), afirst voltage divider having a divider node, circuitry connecting saidvoltage detector input to said divider node, circuitry for applying theoutput of said voltage detector to said base of said transistor,circuitry connecting said collector-emitter circuit and said capacitorin parallel with said voltage (U) to be detected, a first resistanceconnecting the junction of said capacitor and said transistor to saidinput of said voltage detector, and output circuitry connected to saidoutput of said voltage detector for delivering different output signalsin dependence upon whether said voltage detector input receives aconstant signal level or a varying signal level.
 2. The circuitarrangement of claim 1 wherein said output circuitry includes a displaydevice (D) responsive to said voltage detector output such as toindicate whether the detected voltage (U) has reached a specifiedvoltage or lies above or below the specified voltage value.
 3. A circuitarrangement of claim 1 wherein part of said first resistance isadjustable.
 4. The circuit arrangement of claim 1 wherein a portion ofsaid first voltage divider includes a second voltage divider that has adivider node and further including circuitry connecting a referencepotential to said first voltage divider, an integrating capacitorconnected in parallel with said second voltage divider, and a componentconnected to said divider node of said voltage divider for delivering asquare-wave voltage to said output circuitry.
 5. The circuit arrangementof claim 4 wherein said component is part of a microprocessor.
 6. Thecircuit arrangement of claim 1 or claim 4 wherein said output circuitryincludes a microprocessor (C).
 7. The circuit arrangement of claim 4 andfurther including a second resistor connected between said secondvoltage divider and said input of said voltage detector.
 8. The circuitarrangement of claim 8 wherein the pulse duty factor of said square-wavevoltage is variable.
 9. The circuit arrangement of claim 8 whereindifferent values of the voltage (U) are specified by means of varyingpulse duty factors of said square-wave voltage, and further including adisplay device (D) for providing an indication when a particular voltagevalue is reached.
 10. The circuit arrangement of claim 8 wherein saidvoltage (U) to be detected is the voltage of a battery (B), differentvalues of said battery voltage are specified by means of varying pulseduty factors of said square-wave voltage, and further includingcomparing said battery voltage values to a known charging/dischargingcharacteristic of said battery, and adjusting a time-controlledcharge-status indication when a deviation from said charging/dischargingcharacteristic is established.